And the infections don't just go one direction. Pigs can catch human strains as well, and influenza is one of the most costly porcine pathogens for the $19-billion, 113-million-hog U.S. industry. In the past few decades early pig rearing has moved to the south-central and southeastern U.S., from where hogs are shipped to the corn-rich states in the Midwest for fattening and processing. (It's a lot cheaper to ship the pigs to the corn than vice versa, Nelson pointed out at the meeting.) Nelson and a group of colleagues described this process in more detail in a June PLoS Pathogens paper.

For the study, they analyzed the genetics of more than 1,500 hemagglutinin (HA1) sequences collected over the course of seven years. Nelson and her colleagues found that flu in pigs "follows long-distance swine movements from the southern U.S. to the Midwest," with most of the human-origin H1N1 arriving at Midwest hog farms coming from the Southeast, and most of the swine-origin H1N2 coming from the south-central U.S.

And that means the Midwest, as the final destination for many of these pigs, is "likely to provide a reservoir for multiple genetically distinct variants to co-circulate and exchange segments via re-assortment because of the continual importation of swine influenza viruses from other regions," the researchers noted. In other words, the big pig centers in the midwestern parts of the country become pools of various strains of the virus, giving it more opportunities to swap genes and potentially turn into more harmful, more easily transmitted varieties that could go pandemic in people.

Nelson and her colleagues created animations that show the spread of the virus among traveling pigs. The lines show the path of swine influenza H1N2 along common hog routes over time. The swelling circles represent the amount of human-origin influenza in swine population, "with the viral populations increasing first in Oklahoma and then in Minnesota and Iowa," as pigs move from the south-central to the Midwest.

In a second animation, the path of swine influenza H1N1 is traced over time from its origins in the Southeast's hog farms.

"Understanding the importance of long-distance pig transportation in the evolution and spatial dissemination of the influenza virus in swine may inform future strategies for the surveillance and control of influenza," Nelson and her colleagues concluded in their paper.